re PR c++/79681 (ICE with constexpr and bitfield)

	PR c++/79681
	* fold-const.c (make_bit_field_ref): If orig_inner is COMPONENT_REF,
	attempt to use its first operand as BIT_FIELD_REF base.

	* g++.dg/cpp1y/constexpr-79681-1.C: New test.
	* g++.dg/cpp1y/constexpr-79681-2.C: New test.

From-SVN: r245804

gcc/ChangeLog

+2017-03-01  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/79681
+	* fold-const.c (make_bit_field_ref): If orig_inner is COMPONENT_REF,
+	attempt to use its first operand as BIT_FIELD_REF base.
+
 2017-03-01  Richard Biener  <rguenther@suse.de>
 
 	PR middle-end/79721

gcc/fold-const.c

@@ -3862,6 +3862,31 @@ make_bit_field_ref (location_t loc, tree inner, tree orig_inner, tree type,
 {
   tree result, bftype;
 
+  /* Attempt not to lose the access path if possible.  */
+  if (TREE_CODE (orig_inner) == COMPONENT_REF)
+    {
+      tree ninner = TREE_OPERAND (orig_inner, 0);
+      machine_mode nmode;
+      HOST_WIDE_INT nbitsize, nbitpos;
+      tree noffset;
+      int nunsignedp, nreversep, nvolatilep = 0;
+      tree base = get_inner_reference (ninner, &nbitsize, &nbitpos,
+				       &noffset, &nmode, &nunsignedp,
+				       &nreversep, &nvolatilep);
+      if (base == inner
+	  && noffset == NULL_TREE
+	  && nbitsize >= bitsize
+	  && nbitpos <= bitpos
+	  && bitpos + bitsize <= nbitpos + nbitsize
+	  && !reversep
+	  && !nreversep
+	  && !nvolatilep)
+	{
+	  inner = ninner;
+	  bitpos -= nbitpos;
+	}
+    }
+
   alias_set_type iset = get_alias_set (orig_inner);
   if (iset == 0 && get_alias_set (inner) != iset)
     inner = fold_build2 (MEM_REF, TREE_TYPE (inner),

gcc/testsuite/ChangeLog

@@ -5,6 +5,10 @@
 
 2017-03-01  Jakub Jelinek  <jakub@redhat.com>
 
+	PR c++/79681
+	* g++.dg/cpp1y/constexpr-79681-1.C: New test.
+	* g++.dg/cpp1y/constexpr-79681-2.C: New test.
+
 	PR c++/79746
 	* g++.dg/warn/Wunused-parm-9.C: New test.
 

gcc/testsuite/g++.dg/cpp1y/constexpr-79681-1.C

+// PR c++/79681
+// { dg-do compile { target c++14 } }
+// { dg-options "-O2" }
+
+struct A
+{
+  int i : 4;
+};
+
+constexpr bool
+foo ()
+{
+  A x[] = { 1 };
+  return x[0].i;
+}
+
+static_assert (foo(), "");

gcc/testsuite/g++.dg/cpp1y/constexpr-79681-2.C

+// PR c++/79681
+// { dg-do compile { target c++14 } }
+// { dg-options "-O2" }
+
+struct A
+{
+  char i : 4;
+  char k : 1;
+  char l : 3;
+};
+struct B
+{
+  char j : 4;
+};
+struct C
+{
+  long long u;
+  A a[1];
+  B b[1];
+};
+
+constexpr bool
+foo ()
+{
+  C c = { 0, { { 5, 0, 2 } }, { { 6 } } };
+  C d = { 0, { { 6, 0, 1 } }, { { 5 } } };
+  return c.a[0].i == d.a[0].i && c.b[0].j == d.b[0].j;
+}
+
+constexpr bool
+bar ()
+{
+  C c = { 0, { { 5, 0, 2 } }, { { 6 } } };
+  C d = { 0, { { 6, 0, 1 } }, { { 5 } } };
+  return c.a[0].i == d.a[0].i && c.a[0].l == d.a[0].l;
+}
+
+static_assert (foo () == false, "");
+static_assert (bar () == false, "");